1. Field of the Invention
The disclosed invention generally relates to data modems and particularly to clock recovery circuits used in the receivers of data modems. The invention is more particularly directed to gain control for modem envelope clock recovery.
2. Background Art
In synchronous data communications, modems are utilized to transmit information by modulating a carrier signal, and to receive information by demodulating a received signal. The timing in a modem receiver must be appropriately synchronized to the remote transmitter and such timing is based on information extracted from the received signal. A widely used approach to extract timing information includes the detection of the envelope of the modulated signal received by the receiver, which typically involves a non-linear process such as full-wave rectification, multiplication or squaring. Envelope derived timing recovery is commonly used since it is less complex than other techniques, which is an important consideration in microprocessor based modems.
U.S. Pat. No. 3,020,479, issued to Logan on Feb. 6, 1972, isolates the upper and lower sidebands of the received signal, multiplies the respective sidebands and extracts the timing signal from the product.
U.S. Pat. No. 3,524,023, issued to Whang on Aug. 11, 1970, translates the received signal to an intermediate frequency (I.F.), full-wave rectifies the I.F. signal, and filters out the timing signal.
Another technique involves extracting the Nyquist band edge frequencies by bandpass filtering, full wave rectifying the filtered signals, adding the filtered signals, and extracting the timing signal from the sum. This technique is disclosed in a paper "Envelope Derived Timing Recovery in QAM and SQAM systems," D. L. Lyon, IEEE Transactions On Communications, Vol. 23, No. 11, pp. 1327-31, November, 1975.
The detected envelope is utilized in the receiver as a local reference to synchronize the receiver timing to that of the remote transmitter. For example, for a transmitted symbol rate of 2400 symbols per second (baud), the detected envelope would have a frequency of 2400 Hz. Typically, the detected envelope is used as a reference signal for a phase lock loop (PLL) circuit which would provide a symbol rate timing signal at the frequency of the detected envelope and with a fixed phase relation to the detected envelope. The PLL circuit would also provide other timing signals of different frequencies but in synchronism with the symbol rate timing signal. In essence, the detected envelope provides a phase reference for the receiver symbol rate timing and other timing signals. For example, in digital implementations, such other timing signals would include analog-to-digital sample timing and equalizer timing.
Since the envelope is derived from the received signal, it includes distortions caused by the communications link, which may include telephone lines, microwave channels, or satellite channels. The nature and magnitude of such distortions typically vary with time and with the specific communications connections.
Distortion in the detected envelope, particularly amplitude distortion, will adversely affect the dynamic behavior of the timing recovery phase lock loop circuit. As is well known, inappropriate timing signals in the receiver can lead to inefficient or erroneous operation of modem receivers. For example, the effects of improper symbol timing on adaptive equalizers is discussed in "Timing Recovery in Synchronous Equalized Data Communication," IEEE Transactions On Communications, Vol. 23, No. 2, pp. 269-274, February, 1975.
More specifically, variations in the amplitude of the detected envelope as provided to the timing recovery PLL circuit affects the accuracy of its output. For example, if the amplitude of the input to the PLL circuit decreases below its nominal level, loop bandwidth decreases and Q increases, resulting in undesirable overshoot. Moreover, in digitally implemented systems, dynamic range is limited by the number of bits utilized to represent the digital samples. Substantial variations in the amplitude of the detected envelope could cause overflow or underflow.